This invention relates generally to solid state electrical surge suppressors and more particularly to a device for providing failsafe protection for telecommunication equipment with which such suppressors are used.
Solid state surge protection systems conventionally employ a surge protection device having a semiconducting element disposed between a pair of electrodes. This element is arranged in the circuit to selectively conduct electrical energy between the tip line and ground and/or the ring line and ground. For example, in the event a telecommunication circuit experiences an electrical surge as a result of lightning or A.C. line cross or the like in the circuit, the system is designed to shunt voltage from and thereby protect telecommunication equipment connected in the circuit from damage due to the surge condition. On occasion, the circuit may experience an even greater electrical surge which results in destruction of the semiconducting element so that the circuit components and telecommunication equipment would not thereafter be protected against electrical surge conditions. Accordingly, conventional surge protection systems typically incorporate means designed to move the electrodes into engagement with each other following destruction of the semiconducting element to thereby maintain a short circuit condition between the electrodes to protect the circuit and telecommunication equipment coupled to the circuit against subsequently occurring transient surge conditions until such time as the semiconducting element is replaced. An example of such a surge protection device is shown and described in copending application Ser. No. 07/987,038 assigned to the assignee of the present invention.
According to applicable standards, a surge protector when mounted in a telecommunications line protector unit (TLPU) package must meet a number of test requirements including the ability to conduct defined tip to ground and ring to ground currents simultaneously and other defined currents, tip to ground or ring to ground, for specified durations without causing a safety hazard or propagating a fire. At the conclusion of the tests all TLPUs must either be shorted to ground or have a voltage limiting of less than a specified amount at a certain rate of rise.
It is an object of the present invention to provide a failsafe device that can be used with a surge suppressor which will short to ground under any of the required conditions but which will remain unaffected when subjected to less severe fault conditions that the surge suppressor is designed to handle.
Briefly, in accordance with the invention, a failsafe device for use with a telecommunications surge suppressor having electrical leads, including a ground lead, comprises a heat transfer member thermally coupled to the surge suppressor with a shorting bar attached to the heat transfer member by a layer of solder which is chosen to melt at a selected temperature. The shorting bar is aligned with the electrical leads and a force is applied to the shorting bar in the direction of the electrical leads with a tensional force applied to the solder layer so that if the temperature of the solder reaches the melting point due to a fault condition or the like the shorting bar will be released and will be rapidly forced from its first position in engagement with the heat transfer member to a second position in engagement with the electrical leads to thereby provide a short circuit to ground. According to a feature of the invention, the shorting bar is provided with triangular portions bent out of the plane of the bar in order to break through any oxide layer or the like formed on the leads. According to another feature of the invention a triangular groove is formed in a housing member to serve as a guide passage for one of the triangular portions of the shorting bar to control the side-to-side position of the shorting bar as well as to maintain a selected orientation of the bar so that each of the triangular portions will engage a respective lead. According to another feature of the invention, the spring force is applied to the bar at a location to ensure equal distribution of the force on each of the leads. According to another feature of the invention the leads are received in mating grooves between two housing members with one housing member having a support surface for the leads in alignment with the position of the shorting bar. According to yet another feature, a spring is used to provide the shorting force and, in the extended condition, when the shorting bar is in engagement with the leads the spring provides sufficient force, for example, a typical force of approximately 0.75 pounds.